1. Field of the Invention
The present invention relates to an image stabilization apparatus which prevents deterioration of a shot image by correcting an image blur due to shake, and an image pickup apparatus including the image stabilization apparatus.
2. Description of the Related Art
FIG. 18 is a view showing an outline of an image stabilization apparatus included in the conventional camera. A shake which occurs to the camera has six degrees of freedom in total, which are rotational movements of three degrees of freedom constituted of pitching, yawing and rolling movement, and translation movements of three degrees of freedom constituted of movements in an X-axis, a Y-axis and a Z-axis directions. The image stabilization apparatuses which are commercialized at present usually corrects the image blur due to the rotational movements of two degrees of freedom constituted of pitching and yawing movements which significantly influence image stabilization.
Movement of camera is monitored by an angular velocity sensor 130. As the angular velocity sensor, a piezoelectric vibration angular velocity sensor that detects a Coriolis force which is caused by rotation is generally used. The angular velocity sensor 130 contains three detectors which perform detection of pitching movement that is the rotation around the Z-axis in FIG. 18, detection of yawing movement that is the rotation around the Y-axis in FIG. 18, and detection of rolling movement that is the rotation around the X-axis (optical axis) in FIG. 18.
When images blur due to shake is to be corrected, output of the angular velocity sensor 130 is sent to a lens CPU 106, and a target drive position of a correcting lens 101 for image stabilization is calculated. In order to drive the correcting lens 101 to the target drive position, instruction signals are sent to voltage drivers 161x and 161y, and the voltage drivers 161x and 161y follow the instruction signal to drive lens drivers 120x and 120y. The position of the correcting lens 101 is monitored by lens position detectors 110x and 110y, and is fed back to the lens CPU 106. The lens CPU 106 performs positional control of the correcting lens 101 based on the target drive position and the position of the correcting lens 101. By driving the correcting lens according to the shake like this, the image blur caused by shake can be corrected.
However, in the aforementioned image stabilization apparatus, detection of movement of camera due to shake is performed by only the angular velocity sensor 130, and therefore, the angular movement (rotational movement) can be monitored, but movement which causes the optical axis to move parallel vertically or laterally (hereinafter, referred to as parallel movement) cannot be monitored. Accordingly, image stabilization can be performed only for the movements of the two degrees of freedom constituted of pitching and yawing movements.
Here, about the image blur caused by parallel movement, the case of performing shooting by using a micro lens with a focal length of 100 mm will be described as an example. When a landscape at infinity distance is shot by using this lens, if the angular velocity sensor output substantially 0.8 deg/s, the image plane moving velocity is about 1.40 mm/s (=100×sin 0.8) from the focal length. Therefore, the width of the movement of the image plane due to angular movement when shooting with an exposure time of 1/15 second becomes 93 μm (=1.40 mm/15). Further, if the entire camera is moved parallelly in the vertical direction at 1.0 mm/s in addition to the angular movement, shooting is not influenced by the parallel movement velocity component, and an image blur due to parallel movement does not occur, since in the case of infinity shooting, the shooting magnification β is substantially zero.
However, when close-up shooting is performed for shooting a flower or the like, the shooting magnification is very large, and the influence of parallel movement cannot be ignored. For example, when the shooting magnification is equal-magnification (β=1), and the moving velocity in the vertical direction is 1 mm/s, the image on the image plane moves also a moving velocity of 1 mm/s. The movement width in the image plane at the time of performing shooting with an exposure time of 1/15 second becomes 67 μm, and the image blur due to parallel movement cannot be ignored.
Next, a general method (model and mathematical expression) which expresses the movement of an object in a space in a field of physics or engineering will be described. Here, about the model expressing movement of the object on a plane, an ordinary object will be described for facilitating the description. In this case, if the three degrees of freedom of the object are defined, the movement and the position of the object can be uniquely defined.
The first one is the model expressing a translation movement and a rotational movement (see FIGS. 19A and 19B). In a fixed coordinate system O-XY in a plane with the axis of abscissa set as an X-axis and the orthogonal axis set as a Y-axis, the position of the object can be determined if defining the three degrees of freedom: a position X(t) in the X-axis direction; a position Y(t) in the Y-axis direction; and the rotational angle θ(t) of the object itself are specified as shown in FIG. 19A. As shown in FIG. 19B, the movement of the object (velocity vector) can be expressed by three components of an X-axis direction translation velocity Vx(t) and a Y-axis direction translation velocity Vy(t) of a reference point (principal point O2) set on the object, and a rotation angular velocity {dot over (θ)}(t) around the reference point on the object. This model is the commonest.
The second one is the model expressing an instantaneous center of rotation and a rotation radius (see FIG. 20). In the fixed coordinate system O-XY in an XY plane, the object is assumed to be rotating at a rotation velocity {dot over (θ)}(t) with a rotation radius R(t) around a certain point f(t)=(X(t), Y(t)) being set as an instantaneous center of rotation, at a certain instant. Like this, the movement within the plane can be expressed by a locus f(t) of the instantaneous center of rotation and the rotation velocity {dot over (θ)}(t) at the instant. This model is often used in the analysis of a link mechanism in mechanics.
In recent years, cameras equipped with a function of correcting parallel movement are proposed in Japanese Patent Application Laid-Open No. H07-225405 and Japanese Patent Application Laid-Open No. 2004-295027. It can be said that in Japanese Patent Application Laid-Open No. H07-225405, the movement of camera in a three-dimensional space is expressed by a translation movement and a rotation movement based on the measurement values of three accelerometers and three angular velocity sensors.
Further, in Japanese Patent Application Laid-Open No. 2004-295027, in the movement of camera including angular movement and parallel movement, as illustrated in FIG. 2 of Japanese Patent Application Laid-Open No. 2004-295027, a distance n of the rotational center from the focal plane is calculated. In mathematical expression (1) of Japanese Patent Application Laid-Open No. 2004-295027, the angular movement amount which occurs when the focal plane is set as the rotation center is calculated in the first half part, and the parallel movement amount which occurs due to translation movement is calculated in the latter half part. The parallel movement amount of the latter half part is a correction term which is considered by being replaced with rotation in the position alienated from the focal plane by a distance n. The method for obtaining the position n of the rotation center in FIGS. 3A and 3B in Japanese Patent Application Laid-Open No. 2004-295027 uses the concept of an instantaneous center which is frequently used in the mechanics, as the model expressing the movement in the space. This is the idea that the movement in the space can be expressed by a succession of the rotational movement, that is, the movement in the space is a rotational movement of a certain radius with a certain point as the center at the instant, and is the rotational movement with a certain radius with the next certain point as the center at the next instant. Therefore, it can be said that in Japanese Patent Application Laid-Open No. 2004-295027, the movement of camera due to shake is modeled as a succession of the rotational movement having the instantaneous center.
However, the method described in Japanese Patent Application Laid-Open No. H07-225405 has the problem that the calculation amount for obtaining the blur amount in the image plane becomes tremendous, and the algorithm of calculation becomes very complicated. Further, the correction calculation with respect to the optical axis direction blur (out of focus) is not mentioned. Further, it can be said that in Japanese Patent Application Laid-Open No. 2004-295027, movement of camera is modeled as a succession of the rotational movement having the instantaneous center of rotation as described above, and the problem of the model and the mathematical expression is that as described in paragraph [0047] of Japanese Patent Application Laid-Open No. 2004-295027 by itself, in the case of F1≈F2 (the forces applied to two accelerometers), the rotation center position n becomes ∞, and calculation cannot be performed. Further, the fact that the rotation center position n is ∞ means that the movement due to the angle in the pitching direction or in the yawing direction is absent, and this movement cannot be detected by the angular velocity sensor. The correction amount can be calculated by using the output of the two acceleration sensors, but the precision is low and the calculation amount becomes tremendous. Further, by the mathematical expression in this case, the correction calculation of the movement in the optical axis direction cannot be made.
Further, with a change in principal point position of the shooting optical system, the correction error component which will be described later is output from the acceleration sensor (accelerometer), but Japanese Patent Application Laid-Open No. H07-225405 and Japanese Patent Application Laid-Open No. 2004-295027 do not have any corresponding technical disclosure.